At the mammalian neuromuscular junction, the acetylcholine receptor is almost totally localized to the juxtaneural region of the postsynaptic membrane, but the literature suggests the presence of small amounts of receptor in the distal portions of the junctional folds. Studies in this laboratory have shown that a precisely analogous localization probably occurs in electric tissue of electric rays, and that there are small patches of putative receptor in the distal folds. It further appears that receptor localization can be conveniently considered to be the result of a static phase, due to interactions among components within the membrane, and a dynamic phase involving cytoplasmic elements which interact with the membrane. These phases may be studied independently. Thus in one part of the proposed research, it will be determined whether receptor does in fact exist in the distal folds of the endplate, and, if so, whether it is organized in any way, particularly with respect to cytoskeletal components. It is possible that receptor in the distal folds or adjacent sarcolemma is displaced to the juxtaneural membrane during incorporation of newly synthesized receptor into the endplate. A test of this will be undertaken. In the second part, identification of submembrane proteins at the postsynaptic membrane in electroplax will be sought by immunohistochemical localization using antibodies against contractile proteins and tubulin, and by direct isolation of partially native membrane fragments. These fragments, which have been partially purified, appear to contain some of the structurce which adhere to the postsynaptic membrane in situ. If they can be fully purified, they may provide a means to determine how submembrane proteins interact with the receptor. Advantage will also be taken of a second membrane preparation in attempts to perturb and identify the interactions which maintain the receptor localization within the membrane, even in the absence of cellular integrity or of submembrane proteins.